flag: PrintDefaults does not print default of "0" for String flags

[spenczar]

What version of Go are you using (go version)?

go version go1.9.3 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

darwin/amd64

What did you do?

https://play.golang.org/p/F1sa0z0p9V1:

package main

import (
	"flag"
)

var (
	aaa = flag.String("aaa", "", "Set aaa")
	bbb = flag.String("bbb", "0", "Set bbb")
	ccc = flag.String("ccc", "1", "Set ccc")
)

func main() {
	flag.PrintDefaults()
}

What did you expect to see?

  -aaa string
    	Set aaa
  -bbb string
    	Set bbb (default "0")
  -ccc string
    	Set ccc (default "1")

What did you see instead?

  -aaa string
    	Set aaa
  -bbb string
    	Set bbb
  -ccc string
    	Set ccc (default "1")

This is fairly explicit in the relevant code, flag.isZeroValue. The documentation says that "The parenthetical default is omitted if the default is the zero value for the type," but "0" is not the zero value of string.

[garethlewin]

I believe that the switch statement in isZeroValue is not needed, I can't create a reproduction case that fails the top part and needs the switch case.

[ianlancetaylor]

The handling of "0" dates back to https://golang.org/cl/7330 when the isZeroValue function was introduced. It was preserved by https://golang.org/cl/23581 which added the reflect based code.

I would be OK with removing the switch. It's not clear that it's needed any more. CC @robpike

[robpike]

As Ian said, but I think in error, it's NOT clear that it's not needed any more. I do think it's safe to remove, though.

We can try it in 1.11 and see what breaks.

[gopherbot]

Change https://golang.org/cl/103867 mentions this issue: flag: correct zero values when printing defaults